Add Client with response wrapper, request helpers, and full middleware assembly

Implements the top-level httpx.Client that composes the full chain:
  Logging → User Middlewares → Retry → Circuit Breaker → Balancer → Transport

- Response wrapper with JSON/XML/Bytes decoding and body caching
- NewJSONRequest helper with Content-Type and GetBody support
- Functional options: WithBaseURL, WithTimeout, WithRetry, WithEndpoints, etc.
- Integration tests covering retry, balancing, error mapping, and JSON round-trips

client_options.go

@@ -0,0 +1,87 @@
+package httpx
+
+import (
+	"log/slog"
+	"net/http"
+	"time"
+
+	"git.codelab.vc/pkg/httpx/balancer"
+	"git.codelab.vc/pkg/httpx/circuitbreaker"
+	"git.codelab.vc/pkg/httpx/middleware"
+	"git.codelab.vc/pkg/httpx/retry"
+)
+
+type clientOptions struct {
+	baseURL      string
+	timeout      time.Duration
+	transport    http.RoundTripper
+	logger       *slog.Logger
+	errorMapper  ErrorMapper
+	middlewares  []middleware.Middleware
+	retryOpts    []retry.Option
+	enableRetry  bool
+	cbOpts       []circuitbreaker.Option
+	enableCB     bool
+	endpoints    []balancer.Endpoint
+	balancerOpts []balancer.Option
+}
+
+// Option configures a Client.
+type Option func(*clientOptions)
+
+// WithBaseURL sets the base URL prepended to all relative request paths.
+func WithBaseURL(url string) Option {
+	return func(o *clientOptions) { o.baseURL = url }
+}
+
+// WithTimeout sets the overall request timeout.
+func WithTimeout(d time.Duration) Option {
+	return func(o *clientOptions) { o.timeout = d }
+}
+
+// WithTransport sets the base http.RoundTripper. Defaults to http.DefaultTransport.
+func WithTransport(rt http.RoundTripper) Option {
+	return func(o *clientOptions) { o.transport = rt }
+}
+
+// WithLogger enables structured logging of requests and responses.
+func WithLogger(l *slog.Logger) Option {
+	return func(o *clientOptions) { o.logger = l }
+}
+
+// WithErrorMapper sets a function that maps HTTP responses to errors.
+func WithErrorMapper(m ErrorMapper) Option {
+	return func(o *clientOptions) { o.errorMapper = m }
+}
+
+// WithMiddleware appends user middlewares to the chain.
+// These run between logging and retry in the middleware stack.
+func WithMiddleware(mws ...middleware.Middleware) Option {
+	return func(o *clientOptions) { o.middlewares = append(o.middlewares, mws...) }
+}
+
+// WithRetry enables retry with the given options.
+func WithRetry(opts ...retry.Option) Option {
+	return func(o *clientOptions) {
+		o.enableRetry = true
+		o.retryOpts = opts
+	}
+}
+
+// WithCircuitBreaker enables per-host circuit breaking.
+func WithCircuitBreaker(opts ...circuitbreaker.Option) Option {
+	return func(o *clientOptions) {
+		o.enableCB = true
+		o.cbOpts = opts
+	}
+}
+
+// WithEndpoints sets the endpoints for load balancing.
+func WithEndpoints(eps ...balancer.Endpoint) Option {
+	return func(o *clientOptions) { o.endpoints = eps }
+}
+
+// WithBalancer configures the load balancer strategy and options.
+func WithBalancer(opts ...balancer.Option) Option {
+	return func(o *clientOptions) { o.balancerOpts = opts }
+}